British light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a tight ten-round fight at the boxing arena.
But Parker had his moments, connecting with powerful single shots in a fight where the techniques never quite clicked, with excessive holding disrupting the action.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They win," the victor commented in the ring as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to earn a 20th pro success.
Several boxing experts thought the challenger, thirty-one, deserved the decision but the contender endured the second defeat of his 28-bout professional journey.
The Champion Earns Win in Dull Affair
Buatsi's progress has plateaued in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - perhaps in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter willing to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who operated with a tight guard to narrowly win the cleaner rounds.
The audience's tolerance decreased during regular grabbing, with Parker going down various instances during grappling.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding Manchester vs Manchester contest to win the continental title.
The stadium became quiet for a profoundly moving tribute prior to the bout as 10 bells rang in honor of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised match card in the location since Hatton's death. As the last bell faded, the stillness became a roar of "only one Hatton".
The action during the bout equaled the sentiment. The eventual winner was floored by a left hook flush on the face in the early round, getting up unsteadily to his footing on the count of eight.
But the traditional red equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged quickly, noticeable damage developing under his eye as the fighters landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur narrowly won the decision with judges scorecards of draw, narrow lead and 115-112, but each competitor were relentless and courageous, everything their hero the legend represented.
